This application is based on, and claims priority to, Japanese application number 10-70251, filed on Mar. 19, 1998, in Japan, and which is incorporated herein by reference.
This application is also based on, and claims priority to, Japanese application number 10-258114, filed on Sep. 11, 1998, in Japan, and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to cascaded optical amplifiers and, more particularly, to gain and signal level adjustments of cascaded optical amplifiers.
2. Description of the Related Art
Optical communication systems using optical fiber transmission lines are being used to transmit relatively large amounts of information. For this purpose, low-loss (e.g., 0.2 dB/km) optical fibers have been manufactured and used as optical fiber transmission lines. In addition, optical amplifiers are used to compensate for losses in the optical fiber transmission line to thereby allow long-haul transmission.
A conventional optical amplifier includes an optical amplifying medium which is pumped with pump light to provide a gain band. The optical amplifying medium and pump light are chosen so that they provide a gain band which includes a wavelength of signal light. As a result, the signal light will be amplified as the signal light travels through the optical amplifying medium. For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium. A pumping light source supplies pump light having a predetermined wavelength to the EDF. By setting the wavelength of the pump light within a 0.98 xcexcm band or a 1.48 xcexcm band, a gain band including a wavelength band of 1.55 xcexcm can be obtained. Therefore, signal light in the 1.55 xcexcm wavelength band will be amplified.
Another type of conventional optical amplifier has a semiconductor chip as the optical amplifying medium. In this case, pumping is performed by injecting an electric current into the semiconductor chip.
Further, wavelength division multiplexing (WDM) is a known technique for increasing transmission capacity through a single optical fiber. In a system adopting WDM, a plurality of optical carriers having different wavelengths are individually modulated with data. Thus, each modulated carrier represents a channel of the WDM system transmitting an optical signal. The optical signals (that is, the modulated carriers) are then wavelength division multiplexed by an optical multiplexer to obtain WDM signal light. The WDM signal light is then transmitted through an optical fiber transmission line. The WDM signal light is received through the transmission line, and then demultipexed into individual optical signals by an optical demultiplexer. Data can then be detected from these individual optical signals. Therefore, by applying WDM, the transmission capacity of a single optical fiber can be increased in accordance with the number of WDM channels multiplexed together and transmitted through the optical fiber.
When an optical amplifier is inserted along the transmission line in an optical communication system adopting WDM, a transmission distance is limited by the wavelength characteristic of gain which is represented by a gain tilt or gain deviation of the optical amplifier. For example, in an EDFA, it is known that a gain tilt is produced at wavelengths in the vicinity of 1.55 xcexcm, and this gain tilt varies with total input power of signal light and pump light power to the EDFA.
There is an optical amplification device for optical amplification which can maintain the wavelength characteristic of gain constant and obtain a wide input dynamic range. The optical amplification device includes first and second optical amplifiers and a variable optical attenuator optically connected between the first and second optical amplifiers. Automatic gain control (AGC) is applied to each of the first and second optical amplifiers, thereby maintaining constant the wavelength characteristic of gain of each of the first and second optical amplifiers. Further, automatic output level control (ALC) is performed for the second optical amplifier by using the variable optical attenuator, thereby obtaining a wide input dynamic range. That is, the output level of the second optical amplifier is maintained constant irrespective of the input level of the first optical amplifier, so that the input dynamic range of this device is widened.
In such an optical amplification device, AGC is performed so that the gain of the first optical amplifier is maintained constant irrespective of the input level of the first optical amplifier. Accordingly, there arises a problem such that when the power of signal light to be supplied to the first optical amplifier is increased, the power of pump light must be increased by the corresponding amount to increase the output power of the first optical amplifier to provide the required gain. That is, a high-power pumping light source is required for the first optical amplifier to ensure a required input dynamic range.
Accordingly, it is an object of the present invention to provide an optical amplification device and method which does not require a high-power pumping light source.
It is an additional object of the present invention to provide an optical amplification device and method which can maintain the wavelength characteristic of gain constant and can obtain a wide input dynamic range, while requiring a relatively low power pump light.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing objects of the present invention are achieved by providing an apparatus comprising first and second optical amplifiers, and a controller. The first optical amplifier receives a light and amplifies the received light. The second optical amplifier receives the light amplified by the first optical amplifier, and amplifies the received light. The controller, when a level of the light received by the first optical amplifier changes by xcex94, controls a level of the light received by the second optical amplifier to change by approximately xe2x88x92xcex94.
Objects of the present invention are further achieved by providing an apparatus which includes first and second optical amplifiers, a variable attenuator and a controller. The first optical amplifier receives a light and amplifies the received light. The second optical amplifier receives the light amplified by the first optical amplifier, and amplifies the received light. The variable attenuator is optically connected between the first and second optical amplifiers. The controller controls attenuation of the variable attenuator so that, when a level of the light received by the first optical amplifier changes by xcex94, a level of the light received by the second optical amplifier changes by approximately xe2x88x92xcex94.
Objects of the present invention are further achieved by providing an optical communication system. In the optical communication system, optical transmitters transmit optical signals at different wavelengths. A multiplexer multiplexes the optical signals into a wavelength division multiplexed (WDM) signal light. The WDM signal light is transmitted through an optical fiber transmission line. An optical amplification device amplifies the WDM signal light as the WDM signal light is transmitted through the transmission line. The optical amplification device includes first and second optical amplifiers and a controller. The first optical amplifier receives the WDM signal light and amplifies the received WDM signal light. The second optical amplifier receives the WDM signal light amplified by the first optical amplifier, and amplifies the received WDM signal light. The controller, when a level of the WDM signal light received by the first optical amplifier changes by xcex94, controls a level of the WDM signal light received by the second optical amplifier to change by approximately xe2x88x92xcex94.
Objects of the present invention are further achieved by providing an apparatus which includes optical amplifiers optically connected together, each optical amplifier having a corresponding gain. A controller causes the sum of the gains of the optical amplifiers to be constant. There can be two or more optical amplifiers optically connected together in this manner.
In addition, objects of the present invention are achieved by providing an apparatus comprising first and second optical amplifiers connected together, and a controller causing the sum of the gains of the first and second optical amplifiers to be constant. The controller can include a variable attenuator optically connected between the first and second optical amplifiers, where the attenuation of the variable attenuator is controlled to cause the sum of the gains of the first and second optical amplifiers to be constant.
Further, objects of the present invention are achieved by providing an apparatus which includes first and second optical amplifiers, and a gain adjustor. The first optical amplifier amplifies a light with a gain of the first optical amplifier. The second optical amplifier receives the light amplified by the first optical amplifier, and amplifies the received light with a gain of the second optical amplifier. The gain adjustor detects a deviation in gain of the first optical amplifier from a target gain, and adjusts the gain of the second optical amplifier to compensate for the detected deviation.
Objects of the present invention are also achieved by providing an apparatus including a first optical amplifier which amplifies a light with a gain of the first optical amplifier. A first gain controller controls the gain of the first optical amplifier to be constant at a target gain. A second optical amplifier receives the light amplified by the first optical amplifier, and amplifies the received light with a gain of the second optical amplifier. A gain deviation detector detects a deviation in gain of the first optical amplifier from the target gain. A gain adjustor adjusts the gain of the second optical amplifier to compensate for the detected deviation in gain of the first optical amplifier. A level controller can control a level of the light amplified by the first optical amplifier before being amplified by the second optical amplifier to be at a target level, wherein the level controller adjusts the target level to compensate for a detected deviation in gain of the first optical amplifier by the gain deviation detector.
Moreover, objects of the present invention are achieved by providing an optical amplifying device which includes first and second optical amplifiers. The first optical amplifier amplifies a light with a gain of the first optical amplifier. A first gain controller controls the gain of the first optical amplifier to be constant at a target gain for the first optical amplifier. The second optical amplifier receives the light amplified by the first optical amplifier, and amplifies the received light with a gain of the second optical amplifier. A second gain controller controls the gain of the second optical amplifier to be constant at a target gain for the second optical amplifier. A gain deviation detector detects a deviation in gain of the first optical amplifier from the target gain of the first optical amplifier. A gain adjustor adjusts the target gain of the second optical amplifier to compensate for the detected deviation in gain of the first optical amplifier.
Objects of the present invention are also achieved by providing an apparatus including first and second optical amplifiers and a gain adjustor. The first optical amplifier amplifies a light with a gain of the first optical amplifier. The second optical amplifier receives the light amplified by the first optical amplifier, and amplifies the received light with a gain of the second optical amplifier. The gain adjustor detects a deviation in gain of one of the first and second optical amplifiers from a target gain, and adjusts the gain of the other of the first and second optical amplifiers to compensate for the detected deviation.
In addition, objects of the present invention are achieved by providing an apparatus which includes a first optical amplifier amplifying a light with a gain of the first optical amplifier. A first gain controller controls the gain of the first optical amplifier to be constant at a target gain for the first optical amplifier. A second optical amplifier receives the light amplified by the first optical amplifier, and amplifies the received light with a gain of the second optical amplifier. A second gain controller controls the gain of the second optical amplifier to be constant at a target gain for the second optical amplifier. A gain adjustor detects a deviation in gain of one of the first and second optical amplifiers from its target gain, and adjusts the gain of the other of the first and second optical amplifiers to compensate for the detected deviation.
Objects of the present invention are achieved by providing an apparatus which includes a plurality of optical amplifiers cascaded together so that a light is amplified by each optical amplifier as the light travels through the cascaded plurality of optical amplifiers, each optical amplifier amplifying the light with a corresponding gain. A gain adjustor detects a deviation in gain of one of the plurality of optical amplifiers from a target gain, and adjusts the gain of at least one of the other of the plurality of optical amplifiers to compensate for the detected deviation.